Method for manufacturing a golf ball

ABSTRACT

An object of the present invention is to provide a method for manufacturing a golf ball excellent in kneading workability when preparing a core rubber composition containing (d) a carboxylic acid and/or a salt thereof. The present invention provides a method for manufacturing a golf ball comprising the steps of blending (a) a base rubber and at least (b) a carboxylic acid and/or a salt thereof to prepare a first masterbatch; blending (a) a base rubber and at least (c) a crosslinking initiator to prepare a second masterbatch; blending the first masterbatch and second masterbatch to form a core rubber composition; molding the core rubber composition into a spherical core; and forming one or more cover layers covering the spherical core.

FIELD OF THE INVENTION

The present invention relates to a method for manufacturing golf balls,in particular, a technique for manufacturing a spherical core.

DESCRIPTION OF THE RELATED ART

As a method for improving flight distance on driver shots, for example,there are methods of using a core having high resilience and using acore having a hardness distribution in which the hardness increasestoward the surface of the core from the center thereof. The formermethod has an effect of enhancing an initial speed, and the lattermethod has an effect of a higher launch angle and a lower spin rate. Agolf ball having a higher launch angle and a low spin rate travels agreat distance.

For example, Japanese Patent Publications Nos. S61-37178 A, S61-113475A, S61-253079 A, 2008-212681 A, 2008-523952 A and 2009-119256 A disclosetechniques of enhancing resilience of the core. Japanese PatentPublication No. S61-37178 A and S61-113475 A disclose a solid golf ballhaving an inner core where zinc acrylate as a co-crosslinking agent,palmitic acid, stearic acid, or myristic acid as a co-crosslinkingactivator, zinc oxide as another co-crosslinking activator, and areaction rate retarder are blended.

Japanese Patent Publication No. S61-253079 A discloses a solid golf ballformed from a rubber composition containing an α,β-unsaturatedcarboxylic acid in an amount of 15 parts to 35 parts by weight, a metalcompound to react with the α,β-unsaturated carboxylic acid and form asalt thereof in an amount of 7 parts to 60 parts by weight, and a highfatty acid metal salt in an amount of 1 part to 10 parts by weight withrespect to 100 parts by weight of a base rubber.

Japanese Patent Publication No. 2008-212681 A discloses a golf ballcomprising, as a component, a molded and crosslinked product obtainedfrom a rubber composition essentially comprising a base rubber, afiller, an organic peroxide, an α,β-unsaturated carboxylic acid and/or ametal salt thereof, a copper salt of a saturated or unsaturated fattyacid.

Japanese Patent Publication No. 2008-523952 T discloses a golf ball, ora component thereof, molded from a composition comprising a baseelastomer selected from the group consisting of polybutadiene andmixtures of polybutadiene with other elastomers, at least one metallicsalt of an unsaturated monocarboxylic acid, a free radical initiator,and a non-conjugated diene monomer.

Japanese Patent Publication No. 2009-119256 A discloses a method ofmanufacturing a golf ball, comprising preparing a masterbatch of anunsaturated carboxylic acid and/or a metal salt thereof by mixing theunsaturated carboxylic acid and/or the metal salt thereof with a rubbermaterial ahead, using the masterbatch to prepare a rubber compositioncontaining the rubber material, and employing a heated and moldedproduct of the rubber composition as a golf ball component, wherein themasterbatch of the unsaturated carboxylic acid and/or the metal saltthereof comprises; (A) from 20 wt % to 100 wt % of a modifiedpolybutadiene obtained by modifying a polybutadiene having a vinylcontent of from 0 to 2%, a cis-1,4 bond content of at least 80% andactive terminals, the active terminal being modified with at least onetype of alkoxysilane compound, and (B) from 80 wt % to 0 wt % of a dienerubber other than (A) the above rubber component [the figures arerepresented by wt % in the case that a total amount of (A) and (B) equalto 100 wt %] and (C) an unsaturated carboxylic acid and/or a metal saltthereof.

For example, Japanese Patent Publications Nos. H6-154357 A, 2008-194471A, 2008-194473 A and 2010-253268 A disclose a core having a hardnessdistribution. Japanese Patent Publication No. H6-154357 A discloses atwo-piece golf ball comprising a core formed of a rubber compositioncontaining a base rubber, a co-crosslinking agent, and an organicperoxide, and a cover covering said core, wherein the core has thefollowing hardness distribution according to JIS-C type hardness meterreadings: (1) hardness at center: 58-73, (2) hardness at 5 to 10 mm fromcenter: 65-75, (3) hardness at 15 mm from center: 74-82, (4) surfacehardness: 76-84, wherein hardness (2) is almost constant within theabove range, and the relation (1)<(2)<(3)≦(4) is satisfied.

Japanese Patent Publication No.2008-194471 A discloses a solid golf ballcomprising a solid core and a cover layer that encases the core, whereinthe solid core is formed of a rubber composition composed of 100 partsby weight of a base rubber that includes from 60 to 100 parts by weightof a polybutadiene rubber having a cis-1,4 bond content of at least 60%and synthesized using a rare-earth catalyst, from 0.1 to 5 parts byweight of an organosulfur compound, an unsaturated carboxylic acid or ametal salt thereof, an inorganic filler, and an antioxidant; the solidcore has a deformation from 2.0 mm to 4.0 mm, when applying a load froman initial load of 10 kgf to a final load of 130 kgf and has thehardness distribution shown in the following table.

TABLE 1 Shore D Hardness distribution in solid core harness Center 30 to48 Region located 4 mm from center 34 to 52 Region located 8 mm fromcenter 40 to 58 Region located 12 mm from center (Q) 43 to 61 Regionlocated 2 to 3 mm inside of surface (R) 36 to 54 Surface (S) 41 to 59Hardness difference [(Q) − (S)]  1 to 10 Hardness difference [(S) − (R)] 3 to 10

Japanese Patent Publication No. 2008-194473 A discloses a solid golfball comprising a solid core and a cover layer that encases the core,wherein the solid core is formed of a rubber composition composed of 100parts by weight of a base rubber that includes from 60 to 100 parts byweight of a polybutadiene rubber having a cis-1,4 bond content of atleast 60% and synthesized using a rare-earth catalyst, from 0.1 to 5parts by weight of an organosulfur compound, an unsaturated carboxylicacid or a metal salt thereof, and an inorganic filler; the solid corehas a deformation from 2.0 mm to 4.0 mm, when applying a load from aninitial load of 10 kgf to a final load of 130 kgf and has the hardnessdistribution shown in the following table.

TABLE 2 Hardness distribution in solid core Shore D harness Center 25 to45 Region located 5 to 10 mm from center 39 to 58 Region located 15 mmfrom center 36 to 55 Surface (S) 55 to 75 Hardness difference 20 to 50between center and surface

Japanese Patent Publication No. 2010-253268 A discloses a multi-piecesolid golf ball comprising a core, an envelope layer encasing the core,an intermediate layer encasing the envelope layer, and a cover whichencases the intermediate layer and has formed on a surface thereof aplurality of dimples, wherein the core is formed primarily of a rubbermaterial and has a hardness which gradually increases from a center to asurface thereof, the hardness difference in JIS-C hardness units betweenthe core center and the core surface being at least 15 and, letting (I)be the average value for cross-sectional hardness at a position about 15mm from the core center and at the core center and letting (II) be thecross-sectional hardness at a position about 7.5 mm from the corecenter, the hardness difference (I)−(II) in JIS-C units being within ±2;and the envelope layer, intermediate layer and cover have hardness whichsatisfy the condition: cover hardness>intermediate layerhardness>envelope layer hardness.

SUMMARY OF THE INVENTION

The inventors of the present invention have found that a spherical coreformed from a rubber composition containing (a) a base rubber, (b) anα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or ametal salt thereof, (c) a crosslinking initiator, and (d) a specificcarboxylic acid and/or a salt thereof has hardness distribution wherethe hardness increases linearly or almost linearly from a center of thecore toward a surface thereof, and have filed patent applications. Thespherical core having a hardness distribution where the hardnessincreases linearly or almost linearly from the center of the core towardthe surface thereof lowers a spin rate on driver shots, therebyproviding a great flight distance.

The reason why the hardness of the core increases linearly or almostlinearly from the center of the core toward the surface thereof isconsidered as follows. The metal salt of (b) the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms blended in the rubbercomposition is considered to form an ion cluster in the core, therebycrosslinking the rubber molecules with metals. By blending (d) thespecific acid and/or the salt thereof into this rubber composition, (d)the specific acid and/or the salt thereof exchanges a cation with theion cluster formed from the metal salt of (b) the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms, thereby breaking the metalcrosslinking by the metal salt of the α,β-unsaturated carboxylic acidhaving 3 to 8 carbon atoms. This cation exchange reaction easily occursat the core central part where the temperature is high, and less occurstoward the core surface. When molding a core, the internal temperatureof the core is high at the core central part and decreases toward thecore surface, since reaction heat from a crosslinking reaction of thebase rubber accumulates at the core central part. In other words, thebreaking of the metal crosslinking by (d) the specific carboxylic acidand/or the salt thereof easily occurs at the core central part, but lessoccurs toward the surface. As a result, it is conceivable that since acrosslinking density in the core increases from the center of the coretoward the surface thereof, the core hardness increases linearly oralmost linearly from the center of the core toward the surface thereof.

However, when kneading (a) the base rubber, (b) the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms and/or the metal saltthereof, (c) the crosslinking initiator, (d) the carboxylic acid and/orthe salt thereof, and the like with a kneader, there were cases wherethe obtained spherical core did not exhibit the desired hardnessdistribution, because the blend was attached to the wall and the rotorof the kneader. On the other hand, when blending the components using aroll mill, there was a problem of taking a quite long time to blend thecomposition.

The present invention has been achieved in view of the abovecircumstances. An object of the present invention is to provide a methodexcellent in kneading workability in preparing a core rubbercomposition, in a method for manufacturing a golf ball comprising aspherical core formed from the core rubber composition containing (a) abase rubber, (b) an α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms and/or a metal salt thereof, (c) a crosslinking initiator, (d) acarboxylic acid and/or a salt thereof, and (e) a metal compound wherenecessary, and at least one cover layer covering the spherical core.

The present invention provides a method for manufacturing a golf ballthat comprises a spherical core formed from a core rubber compositioncontaining (a) a base rubber, (b) an α,β-unsaturated carboxylic acidhaving 3 to 8 carbon atoms and/or a metal salt thereof as aco-crosslinking agent, (c) a crosslinking initiator, (d) a carboxylicacid and/or a salt thereof, and (e) a metal compound where necessary,and at least one cover layer covering the spherical core, comprising thesteps of: blending (a) the base rubber and at least (d) the carboxylicacid and/or the salt thereof to prepare a first masterbatch; blending(a) the base rubber and at least (c) the crosslinking initiator toprepare a second masterbatch; blending the first masterbatch and thesecond masterbatch to prepare the core rubber composition; molding thecore rubber composition into the spherical core; and forming at leastone cover layer covering the spherical core.

A gist of the present invention resides in a point of blending (a) thebase rubber and at least (d) the carboxylic acid and/or the salt thereofto prepare a first masterbatch; blending (a) the base rubber and atleast (c) the crosslinking initiator to prepare a second masterbatch;blending the first masterbatch and second masterbatch to prepare thecore rubber composition; and molding the core rubber composition intothe spherical core. That is, (d) the carboxylic acid and/or the saltthereof and (c) the crosslinking initiator are separately blended into(a) the base rubber respectively to prepare the first masterbatch andsecond masterbatch, then the first masterbatch and second masterbatchare blended, thereby improving kneading workability of the core rubbercomposition.

According to the present invention, it is possible to provide a methodexcellent in kneading workability in preparing a core rubbercomposition, in a method for manufacturing a golf ball comprising aspherical core formed from the core rubber composition containing (a) abase rubber, (b) an α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms and/or a metal salt thereof, (c) a crosslinking initiator, (d) acarboxylic acid and/or a salt thereof, and (e) a metal compound wherenecessary, and at least one cover layer covering the spherical core.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cutaway view of the golf ball of the preferredembodiment of the present invention;

FIG. 2 is a graph showing the hardness distribution of the core;

FIG. 3 is a graph showing the hardness distribution of the core;

FIG. 4 is a graph showing the hardness distribution of the core;

FIG. 5 is a graph showing the hardness distribution of the core;

FIG. 6 is a graph showing the hardness distribution of the core;

FIG. 7 is a graph showing the hardness distribution of the core;

FIG. 8 is a graph showing the hardness distribution of the core;

FIG. 9 is a graph showing the hardness distribution of the core; and

FIG. 10 is a graph showing the hardness distribution of the core.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a method for manufacturing a golf ballthat comprises a spherical core formed from a core rubber compositioncontaining (a) a base rubber, (b) an α,β-unsaturated carboxylic acidhaving 3 to 8 carbon atoms and/or a metal salt thereof as aco-crosslinking agent, (c) a crosslinking initiator, (d) a carboxylicacid and/or a salt thereof, and (e) a metal compound where necessary,and at least one cover layer covering the spherical core, comprising thesteps of: blending (a) the base rubber and at least (d) the carboxylicacid and/or the salt thereof to prepare a first masterbatch; blending(a) the base rubber and at least (c) the crosslinking initiator toprepare a second masterbatch; blending the first masterbatch and thesecond masterbatch to prepare the core rubber composition; molding thecore rubber composition into the spherical core; and forming at leastone cover layer covering the spherical core.

The core rubber composition used in the preset invention will bedescribed. In the method for manufacturing the golf ball of the presentinvention, the spherical core is formed from the core rubber compositioncontaining (a) the base rubber, (b) the α,β-unsaturated carboxylic acidhaving 3 to 8 carbon atoms and/or the metal salt thereof as theco-crosslinking agent, (c) the crosslinking initiator, (d) thecarboxylic acid and/or the salt thereof, and (e) the metal compoundwhere necessary.

First, (a) the base rubber used in the present invention will bedescribed. As (a) the base rubber used in the present invention, naturalrubber and/or synthetic rubber can be used. For example, polybutadienerubber, natural rubber, polyisoprene rubber, styrene polybutadienerubber, ethylene-propylene-diene rubber (EPDM), or the like can be used.These rubbers may be used solely or two or more of these rubbers may beused in combination. Typically preferred of them is the highcis-polybutadiene having a cis-1,4 bond in a proportion of 40% or more,more preferably 80% or more, even more preferably 90% or more in view ofits superior resilience property.

The high-cis polybutadiene preferably has a 1,2-vinyl bond in a contentof 2 mass % or less, more preferably 1.7 mass % or less, and even morepreferably 1.5 mass % or less. If the content of 1,2-vinyl bond isexcessively high, the resilience may be lowered.

The high-cis polybutadiene is preferably one synthesized using a rareearth element catalyst. When a neodymium catalyst, which employs aneodymium compound which is a lanthanum series rare earth elementcompound, is used, a polybutadiene rubber having a high content of acis-1,4 bond and a low content of a 1,2-vinyl bond is obtained withexcellent polymerization activity. Such a polybutadiene rubber isparticularly preferred.

The high-cis polybutadiene preferably has a Mooney viscosity (ML₁₊₄(100° C.)) of 30 or more, more preferably 32 or more, even morepreferably 35 or more, and preferably has a Mooney viscosity (ML₁₊₄(100° C.)) of 140 or less, more preferably 120 or less, even morepreferably 100 or less, and most preferably 80 or less. It is noted thatthe Mooney viscosity (ML₁₊₄ (100° C.)) in the present invention is avalue measured according to JIS K6300 using an L rotor under theconditions of: a preheating time of 1 minute; a rotor revolution time of4 minutes; and a temperature of 100° C.

The high-cis polybutadiene preferably has a molecular weightdistribution Mw/Mn (Mw: weight average molecular weight, Mn: numberaverage molecular weight) of 2.0 or more, more preferably 2.2 or more,even more preferably 2.4 or more, and most preferably 2.6 or more, andpreferably has a molecular weight distribution Mw/Mn of 6.0 or less,more preferably 5.0 or less, even more preferably 4.0 or less, and mostpreferably 3.4 or less. If the molecular weight distribution (Mw/Mn) ofthe high-cis polybutadiene is excessively low, the processabilitydeteriorates. If the molecular weight distribution (Mw/Mn) of thehigh-cis polybutadiene is excessively high, the resilience may belowered. It is noted that the measurement of the molecular weightdistribution is conducted by gel permeation chromatography(“HLC-8120GPC”, manufactured by Tosoh Corporation) using a differentialrefractometer as a detector under the conditions of column: GMHHXL(manufactured by Tosoh Corporation), column temperature: 40° C., andmobile phase: tetrahydrofuran, and calculated by converting based onpolystyrene standard.

Next, (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atomsand/or a metal salt thereof will be described. (b) The α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereofis blended as a co-crosslinking agent in the rubber composition and hasan action of crosslinking a rubber molecule by graft polymerization to abase rubber molecular chain. In the case that the rubber compositionused in the present invention contains only the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms as the co-crosslinking agent,the rubber composition further contains (e) the metal compound whichwill be described later as an essential component. Neutralizing theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms with themetal compound in the rubber composition provides substantially the sameeffect as using the metal salt of the α,β-unsaturated carboxylic acidhaving 3 to 8 carbon atoms. Further, in the case of using theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and the metalsalt thereof in combination, (e) the metal compound may be used as anoptional component.

The α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms includes,for example, acrylic acid, methacrylic acid, fumaric acid, maleic acid,crotonic acid, and the like.

Examples of the metals constituting the metal salts of theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms include:monovalent metal ions such as sodium, potassium, lithium or the like;divalent metal ions such as magnesium, calcium, zinc, barium, cadmium orthe like; trivalent metal ions such as aluminum ion or the like; andother metal ions such as zirconium or the like. The above metal ions canbe used solely or as a mixture of at least two of them. Of these metalions, divalent metal ions such as magnesium, calcium, zinc, barium,cadmium or the like are preferable. Use of the divalent metal salts ofthe α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms easilygenerates a metal crosslinking between the rubber molecules. Especially,as the divalent metal salt, zinc acrylate is preferable, because zincacrylate enhances the resilience of the resultant golf ball. Theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or ametal salt thereof may be used solely or in combination at least two ofthem.

The content of (b) the α,β-unsaturated carboxylic acid having 3 to 8carbon atoms and/or the metal salt thereof is preferably 15 parts bymass or more, more preferably 20 parts by mass or more, and ispreferably 50 parts by mass or less, more preferably 45 parts by mass orless, even more preferably 35 parts by mass or less, with respect to 100parts by mass of (a) the base rubber. If the content of (b) theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or themetal salt thereof is less than 15 parts by mass, the content of (c) thecrosslinking initiator which will be described below must be increasedin order to obtain the appropriate hardness of the constituting memberformed from the rubber composition, which tends to cause the lowerresilience. On the other hand, if the content of (b) the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereofexceeds 50 parts by mass, the constituting member formed from the rubbercomposition becomes excessively hard, which tends to cause the lowershot feeling.

(c) The crosslinking initiator is blended in order to crosslink (a) thebase rubber component. As (c) the crosslinking initiator, an organicperoxide is preferred. Specific examples of the organic peroxide includeorganic peroxides such as dicumyl peroxide,1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane,2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide. Theseorganic peroxides may be used solely or two or more of these organicperoxides may be used in combination. Dicumyl peroxide is preferablyused of them.

The content of (c) the crosslinking initiator is preferably 0.2 part bymass or more, and more preferably 0.5 part by mass or more, and ispreferably 5.0 parts by mass or less, and more preferably 2.5 parts bymass or less, with respect to 100 parts by mass of (a) the base rubber.If the content of (c) the crosslinking initiator is less than 0.2 partby mass, the constituting member formed from the rubber compositionbecomes too soft, and thus the golf ball may have the lower resilience.If the content of (c) the crosslinking initiator exceeds 5.0 parts bymass, the amount of (b) the co-crosslinking agent must be decreased inorder to obtain the appropriate hardness of the constituting memberformed from the rubber composition, resulting in the insufficientresilience and lower durability of the golf ball.

(d) The carboxylic acid and/or the salt thereof used in the presentinvention will be described. It is though that (d) the carboxylic acidand/or the salt thereof has an action of breaking the metal crosslinkingby the metal salt of (b) the α,β-unsaturated carboxylic acid having 3 to8 carbon atoms, in the center part of the core, when molding the core.

(d) The carboxylic acid and/or the salt thereof may include any one ofan aliphatic carboxylic acid (sometimes may be merely referred to as“fatty acid” in the present invention) and/or a salt thereof and anaromatic carboxylic acid and/or a salt thereof; however, the aliphaticcarboxylic acid and/or the salt thereof is preferred. The carboxylicacid preferably includes a carboxylic acid having 4 to 30 carbon atomsand/or a salt thereof, more preferably a carboxylic acid having 5 to 25carbon atoms and/or a salt thereof. (d) The carboxylic acid and/or thesalt thereof does not include (b) the α,β-unsaturated carboxylic acidhaving 3 to 8 carbon atoms and/or the metal salt thereof as theco-crosslinking agent.

The fatty acid may be either a saturated fatty acid or an unsaturatedfatty acid; however, a saturated fatty acid is preferable. Specificexamples of the saturated fatty acids (IUPAC name) are butanoic acid(C4), pentanoic acid (C5), hexanoic acid (C6), heptanoic acid (C7),octanoic acid (C8), nonanoic acid (C9), decanoic acid (C10), undecanoicacid (C11), dodecanoic acid (C12), tridecanoic acid (C13), tetradecanoicacid (C14), pentadecanoic acid (C15), hexadecnoic acid (C16),heptadecanoic acid (C17), octadecanoic acid (C18), nonadecanoic acid(C19), icosanoic acid (C20), henicosanoic acid (C21), docosanoic acid(C22), tricosanoic acid (C23), tetracosanoic acid (C24), pentacosanoicacid (C25), hexacosanoic acid (C26), heptacosanoic acid (C27),octacosanoic acid (C28), nonacosanoic acid (C29), triacontanoic acid(C30).

Specific examples of the unsaturated fatty acid (IUPAC) are butenoicacid (C4), pentenoic acid (C5), hexenoic acid (C6), heptenoic acid (C7),octenoic acid (C8), nonenoic acid (C9), decenoic acid (C10), undecenoicacid (C11), dodecenoic acid (C12), tridecenoic acid (C13), tetradecenoicacid (C14), pentadecenoic acid (C15), hexadecenoic acid (C16),heptadecenoic acid (C17), octadecenoic acid (C18), nonadecenoic acid(C19), icosenoic acid (C20), henicosenoic acid (C21), docosenoic acid(C22), tricosenoic acid (C23), tetracosenoic acid (C24), pentacosenoicacid (C25), hexacosenoic acid (C26), heptacosenoic acid (C27),octacosenoic acid (C28), nonacosenoic acid (C29), triacontenoic acid(C30).

Specific examples of the fatty acid (Common name) are, butyric acid(C4), valeric acid (C5), caproic acid (C6), enanthic acid (C7), caprylicacid (C8), pelargonic acid (C9), capric acid (C10), lauric acid (C12),myristic acid (C14), myristoleic acid (C14), pentadecylic acid (C15),palmitic acid (C16), palmitoleic acid (C16), margaric acid (C17),stearic acid (C18), elaidic acid (C18), vaccenic acid (C18), oleic acid(C18), linoleic acid (C18), linolenic acid (C18), 12-hydroxystearic acid(C18), arachidic acid (C20), gadoleic acid (C20), arachidonic acid(C20), eicosenoic acid (C20), behenic acid (C22), erucic acid (C22),lignoceric acid (C24), nervonic acid (C24), cerotic acid (C26), montanicacid (C28), and melissic acid (C30). The fatty acid may be used alone oras a mixture of at least two of them. Of those described above, capricacid, lauric acid, myristic acid, palmitic acid, stearic acid, behenicacid and oleic acid are preferable as the fatty acid.

There is no particular limitation on the aromatic carboxylic acid, aslong as it is a compound that has an aromatic ring and a carboxyl group.Specific examples of the aromatic carboxylic acid include, for example,benzoic acid (C7), phthalic acid (C8), isophthalic acid (C8),terephthalic acid (C8), hemimellitic acid (benzene-1,2,3-tricarboxylicacid) (C9), trimellitic acid (benzene-1,2,4-tricarboxylic acid) (C9),trimesic acid (benzene-1,3,5-tricarboxylic acid) (C9), mellophanic acid(benzene-1,2,3,4-tetracarboxylic acid) (C10), prehnitic acid(benzene-1,2,3,5-tetracarboxylic acid) (C10), pyromellitic acid(benzene-1,2,4,5-tetracarboxylic acid) (C10), mellitic acid (benzenehexacarboxylic acid) (C12), diphenic acid (biphenyl-2,2′-dicarboxylicacid) (C12), toluic acid (methylbenzoic acid) (C8), xylic acid (C9),prehnitylic acid (2,3,4-trimethylbenzoic acid) (C10), γ-isodurylic acid(2,3,5-trimethylbenzoic acid) (C10), durylic acid(2,4,5-trimethylbenzoic acid) (C10), β-isodurylic acid(2,4,6-trimethylbenzoic acid) (C10), α-isodurylic acid(3,4,5-trimethylbenzoic acid) (C10), cuminic acid (4-isopropylbenzoicacid) (C10), uvitic acid (5-methylisophthalic acid) (C9), α-toluic acid(phenylacetic acid) (C8), hydratropic acid (2-phenylpropanoic acid)(C9), and hydrocinnamic acid (3-phenylpropanoic acid) (C9).

Furthermore, examples of the aromatic carboxylic acid substituted with ahydroxyl group, an alkoxy group, or an oxo group include, for example,salicylic acid (2-hydroxybenzoic acid) (C7), anisic acid (methoxybenzoicacid) (C8), cresotinic acid (hydroxy(methyl)benzoic acid) (C8),o-homosalicylic acid (2-hydroxy-3-methylbenzoic acid) (C8),m-homosalicylic acid (2-hydroxy-4-methylbenzoic acid) (C8),p-homosalicylic acid (2-hydroxy-5-methylbenzoic acid) (C8),o-pyrocatechuic acid (2,3-dihydroxybenzoic acid) (C7), β-resorcylic acid(2,4-dihydroxybenzoic acid) (C7), γ-resorcylic acid(2,6-dihydroxybenzoic acid) (C7), protocatechuic acid(3,4-dihydroxybenzoic acid) (C7), α-resorcylic acid(3,5-dihydroxybenzoic acid) (C7), vanillic acid(4-hydroxy-3-methoxybenzoic acid) (C8), isovanillic acid(3-hydroxy-4-methoxybenzoic acid) (C8), veratric acid(3,4-dimethoxybenzoic acid) (C9), o-veratric acid (2,3-dimethoxybenzoicacid) (C9), orsellinic acid (2,4-dihydroxy-6-methylbenzoic acid) (C8),m-hemipinic acid (4,5-dimethoxyphthalic acid) (C10), gallic acid(3,4,5-trihydroxybenzoic acid) (C7), syringic acid(4-hydroxy-3,5-dimethoxybenzoic acid) (C9), asaronic acid(2,4,5-trimethoxybenzoic acid) (C10), mandelic acid(hydroxy(phenyl)acetic acid) (C8), vanilmandelic acid(hydroxy(4-hydroxy-3-methoxy phenyl)acetic acid) (C9), homoanisic acid((4-methoxy phenyl)acetic acid) (C9), homogentisic acid((2,5-dihydroxyphenyl) acetic acid) (C8), homoprotocatechuic acid((3,4-dihydroxyphenyl) acetic acid) (C8), homovanillic acid((4-hydroxy-3-methoxy phenyl) acetic acid) (C9), homoisovanillic acid((3-hydroxy-4-methoxy phenyl) acetic acid) (C9), homoveratric acid((3,4-dimethoxy phenyl)acetic acid) (C10), o-homoveratric acid((2,3-dimethoxy phenyl)acetic acid) (C10), homophthalic acid(2-(carboxymethyl)benzoic acid) (C9), homoisophthalic acid(3-(carboxymethyl) benzoic acid) (C9), homoterephthalic acid(4-(carboxymethyl)benzoic acid) (C9), phthalonic acid(2-(carboxycarbonyl)benzoic acid) (C9), isophthalonic acid(3-(carboxycarbonyl) benzoic acid) (C9), terephthalonic acid(4-(carboxycarbonyl) benzoic acid) (C9), benzilic acid (hydroxydiphenylacetic acid) (C14), atrolactic acid (2-hydroxy-2-phenylpropanoicacid) (C9), tropic acid (3-hydroxy-2-phenylpropanoic acid) (C9),melilotic acid (3-(2-hydroxyphenyl)propanoic acid) (C9), phloretic acid(3-(4-hydroxy phenyl)propanoic acid) (C9), hydrocaffeic acid(3-(3,4-dihydroxyphenyl)propanoic acid) (C9), hydroferulic acid(3-(4-hydroxy-3-methoxy phenyl)propanoic acid) (C10), hydroisoferulicacid (3-(3-hydroxy-4-methoxy phenyl) propanoic acid) (C10), p-coumaricacid (3-(4-hydroxy phenyl) acrylic acid) (C9), umbellic acid(3-(2,4-dihydroxyphenyl)acrylic acid) (C9), caffeic acid(3-(3,4-dihydroxyphenyl)acrylic acid) (C9), ferulic acid(3-(4-hydroxy-3-methoxy phenyl)acrylic acid) (C10), isoferulic acid(3-(3-hydroxy-4-methoxy phenyl)acrylic acid) (C10), and sinapic acid(3-(4-hydroxy-3,5-dimethoxy phenyl) acrylic acid) (C11).

The salt of (d) the carboxylic acid may include a salt of the carboxylicacids described above. The cation component of the salt of thecarboxylic acid may be any one of a metal ion, an ammonium ion and anorganic cation. The metal ion includes monovalent metal ions such assodium, potassium, lithium, silver and the like; divalent metal ionssuch as magnesium, calcium, zinc, barium, cadmium, copper, cobalt,nickel, manganese and the like; trivalent metal ions such as aluminum,iron and the like; and other ions such as tin, zirconium, titanium andthe like. The cation components may be used alone or as a mixture of atleast two of them.

The organic cation includes a cation having a carbon chain. The organiccation includes, for example, without limitation, an organic ammoniumion. Examples of the organic ammonium ion are: primary ammonium ionssuch as stearyl ammonium ion, hexyl ammonium ion, octhyl ammonium ion,2-ethyl hexyl ammonium ion or the like; secondary ammonium ions such asdodecyl(lauryl)ammonium ion, octadecyl(stearyl)ammonium ion or the like;tertiary ammonium ions such as trioctyl ammonium ion or the like; andquaternary ammonium ions such as dioctyldimethyl ammonium ion,distearyldimethyl ammonium ion or the like. Those organic cation may beused alone or as a mixture of at least two of them.

The content of (d) the carboxylic acid and/or the salt thereof ispreferably 0.1 part by mass or more, more preferably 0.5 part by mass ormore, more preferably 1.0 part by mass or more, and is preferably 40.0parts by mass or less, more preferably 30.0 parts by mass or less, evenmore preferably 20.0 parts by mass or less with respect to 100 parts bymass of (a) the base rubber.

If the content of (d) the carboxylic acid and/or the salt thereof is toolittle, an effect of adding (d) the carboxylic acid and/or the saltthereof is not sufficient, and thus the degree of the outer-hardinner-soft structure of the spherical core may be lowered. If thecontent is too much, the resilience of the core may be lowered, sincethe hardness of the resultant core may be lowered as a whole. There arecases where the surface of the zinc acrylate used as the co-crosslinkingagent is treated with a carboxylic acid and/or a salt thereof to improvethe dispersibility to the rubber. In the case of using zinc acrylatewhose surface is treated with a carboxylic acid and/or a salt thereof,in the present invention, the amount of the carboxylic acid and/or thesalt thereof used as a surface treating agent is included in the contentof (d) the carboxylic acid and/or the salt thereof. For example, if 25parts by mass of zinc acrylate whose surface treatment amount with thecarboxylic acid and/or the salt thereof is 10 mass % is used, the amountof the carboxylic acid and/or the salt thereof is 2.5 parts by mass andthe amount of zinc acrylate is 22.5 parts by mass. Thus, 2.5 parts bymass is counted as the content of (d) the carboxylic acid and/or thesalt thereof.

The rubber composition used in the present invention further contains(e) a metal compound, where necessary. (e) The metal compound is afiller to improve properties of the core rubber composition. Forexample, (e) the metal compound is used, without limitation, as aneutralizing agent to neutralize the α,β-unsaturated carboxylic acidhaving 3 to 8 carbon atoms in the case of containing only (b) theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as theco-crosslinking agent, a weight adjusting agent to adjust the weight ofthe spherical core, a hardness adjusting agent to adjust the hardness ofthe spherical core, or an inorganic pigment. (e) The metal compound maybe used for either one purpose or multiple purposes.

(e) The metal compound that can neutralize (b) the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms as the co-crosslinking agentincludes, for example, metal hydroxides such as magnesium hydroxide,zinc hydroxide, calcium hydroxide, sodium hydroxide, lithium hydroxide,potassium hydroxide, copper hydroxide, and the like; metal oxides suchas magnesium oxide, calcium oxide, zinc oxide, copper oxide, and thelike; metal carbonates such as magnesium carbonate, zinc carbonate,calcium carbonate, sodium carbonate, lithium carbonate, potassiumcarbonate, and the like. In light of reacting with (b) theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as theco-crosslinking agent to form a metal crosslinking, (e) the metalcompound preferably includes a divalent metal compound, more preferablyincludes a zinc compound. Use of the zinc compound provides a golf ballwith excellent resilience. The content of (e) the metal compound used asthe neutralizing agent is preferably determined in accordance with themole number of the carboxyl group of (b) the α,β-unsaturated carboxylicacid having 3 to 8 carbon atoms as well as the desired degree ofneutralization.

(e) The metal compound used as the filler to adjust the weight andhardness of the spherical core includes, for example, zinc oxide, bariumsulfate, calcium carbonate, magnesium oxide, tungsten powder, molybdenumpowder, or the like. Preferred is zinc oxide as (e) the metal compoundused as the filler to adjust the weight and hardness of the sphericalcore. It is considered that zinc oxide functions as a vulcanizationactivator and increases the hardness of the entire core. The content ofthe metal compound used as the filler is preferably 0.5 part by mass ormore, more preferably 1 part by mass or more, and is preferably 30 partsby mass or less, more preferably 25 parts by mass or less, even morepreferably 20 parts by mass or less with respect to 100 parts by mass ofthe base rubber.

The metal compounds may be used solely or in combination of at least twoof them.

The core rubber composition preferably further contains (f) an organicsulfur compound. In the present invention, by using (f) the organicsulfur compound and (d) the carboxylic acid and/or the salt thereof incombination for the core rubber composition, the degree of theouter-hard and inner-soft structure of the core can be controlled, whilemaintaining approximate linearity of the core hardness distribution. (f)The organic sulfur compound is not particularly limited, as long as itis an organic compound having a sulfur atom in the molecule thereof.Examples thereof include an organic compound having a thiol group (—SH),a polysulfide bond having 2 to 4 sulfur atoms (—S—S—, —S—S—S—, or—S—S—S—S—), or a metal salt thereof (—SM, —S-M-S—, —S-M-S—S—,—S—S-M-S—S—, —S-M-S—S—S—, or the like; M is a metal atom). Furthermore,(f) the organic sulfur compound may be any one of aliphatic compounds(aliphatic thiol, aliphatic thiocarboxylic acid, aliphaticdithiocarboxylic acid, aliphatic polysulfides, or the like),heterocyclic compounds, alicyclic compounds (alicyclic thiol, alicyclicthiocarboxylic acid, alicyclic dithiocarboxylic acid, alicyclicpolysulfides, or the like), and aromatic compounds. (f) The organicsulfur compound includes, for example, thiophenols, thionaphthols,polysulfides, thiocarboxylic acids, dithiocarboxylic acids,sulfenamides, thiurams, dithiocarbamates, and thiazoles. From the aspectof the larger hardness distribution of the spherical core, (f) theorganic sulfur compound preferably includes, organic compounds having athiol group (—SH) or a metal salt thereof, more preferably thiophenols,thionaphthols, or a metal salt thereof. Examples of the metal salts aresalts of monovalent metals such as sodium, lithium, potassium, copper(I), and silver (I), and salts of divalent metals such as zinc,magnesium, calcium, strontium, barium, titanium (II), manganese (II),iron (II), cobalt (II), nickel(II), zirconium(II), and tin (II).

Examples of the thiophenols include, for example, thiophenol;thiophenols substituted with a fluoro group, such as 4-fluorothiophenol,2,5-difluorothiophenol, 2,4,5-trifluorothiophenol,2,4,5,6-tetrafluorothiophenol, pentafluorothiophenol and the like;thiophenols substituted with a chloro group, such as 2-chlorothiophenol,4-chlorothiophenol, 2,4-dichlorothiophenol, 2,5-dichlorothiophenol,2,6-dichlorothiophenol, 2,4,5-trichlorothiophenol,2,4,5,6-tetrachlorothiophenol, pentachlorothiophenol and the like;thiophenols substituted with a bromo group, such as 4-bromothiophenol,2,5-dibromothiophenol, 2,4,5-tribromothiophenol,2,4,5,6-tetrabromothiophenol, pentabromothiophenol and the like;thiophenols substituted with an iodo group, such as 4-iodothiophenol,2,5-diiodothiophenol, 2,4,5-triiodothiophenol,2,4,5,6-tetraiodothiophenol, pentaiodothiophenol and the like; or ametal salt thereof. As the metal salt, zinc salt is preferred.

Examples of the naphthalenethiols (thionaphthols) are2-naphthalenethiol, 1-naphthalenethiol, 2-chloro-1-naphthalenethiol,2-bromo-1-naphthalenethiol, 2-fluoro-1-naphthalenethiol,2-cyano-1-naphthalenethiol, 2-acetyl-1-naphthalenethiol,1-chloro-2-naphthalenethiol, 1-bromo-2-naphthalenethiol,1-fluoro-2-naphthalenethiol, 1-cyano-2-naphthalenethiol, and1-acetyl-2-naphthalenethiol and metal salts thereof. Preferable examplesinclude 1-naphthalenethiol, 2-naphthalenethiol and zinc salt thereof.

The sulfenamide based organic sulfur compound includes, for example,N-cyclohexyl-2-benzothiazole sulfenamide,N-oxydiethylene-2-benzothiazole sulfenamide, andN-t-butyl-2-benzothiazole sulfenamide. The thiuram based organic sulfurcompound includes, for example, tetramethylthiuram monosulfide,tetramethylthiuram disulfide, tetraethylthiuram disulfide,tetrabutylthiuram disulfide, and dipentamethylenethiuram tetrasulfide.The dithiocarbamates include, for example, zinc dimethyldithiocarbamate,zinc diethyldithiocarbamate, zinc dibutyldithiocarbamate, zincethylphenyl dithiocarbamate, sodium dimethyldithiocarbamate, sodiumdiethyldithiocarbamate, copper (II) dimethyldithiocarbate, iron (III)dimethyldithiocarbamate, selenium diethyldithiocarbamate, and telluriumdiethyldithiocarbamate. The thiazole based organic sulfur compoundincludes, for example, 2-mercaptobenzothiazole (MBT), dibenzothiazyldisulfide (MBTS), sodium salt, zinc salt, copper salt, orcyclohexylamine salt of 2-mercaptobenzothiazole,2-(2,4-dinitrophenyl)mercaptobenzothiazole, and2-(2,6-diethyl-4-morpholinothio)benzothiazole.

(f) The organic sulfur compound can be used solely or as a mixture of atleast two of them.

The content of (f) the organic sulfur compound is preferably 0.05 partby mass or more, more preferably 0.1 part by mass or more, and ispreferably 5.0 parts by mass or less, more preferably 2.0 parts by massor less with respect to 100 parts by mass of (a) the base rubber. If thecontent of (f) the organic sulfur compound is less than 0.05 part bymass, an effect of adding (f) the organic sulfur compound cannot beobtained and thus the resilience may not improve. If the content of (f)the organic sulfur compound exceeds 5.0 parts by mass, the compressiondeformation amount of the obtained golf ball becomes large and thus theresilience may be lowered.

The rubber composition used in the present invention can further includeadditives such as an antioxidant, a peptizing agent, and a softener.Further, as described above, if the rubber composition used in thepresent invention contains only the α,β-unsaturated carboxylic acidhaving 3 to 8 carbon atoms as a crosslinking agent, the rubbercomposition preferably contains (e) the metal compound.

The content of the antioxidant is preferably 0.1 part by mass or moreand 1 part by mass or less with respect to 100 parts by mass of (a) thebase rubber. The content of the peptizing agent is preferably 0.1 partby mass or more and 5 parts by mass or less with respect to 100 parts bymass of (a) the base rubber.

The present invention provides the method for manufacturing the golfball that comprises the spherical core formed from the above core rubbercomposition and at least one cover layer covering the spherical core,comprising the steps of blending (a) the base rubber and at least (d)the carboxylic acid and/or the salt thereof to prepare a firstmasterbatch; blending (a) the base rubber and at least (c) thecrosslinking initiator to prepare a second masterbatch; blending thefirst masterbatch and the second masterbatch to prepare the core rubbercomposition; molding the core rubber composition into the sphericalcore; and forming at least one cover layer covering the spherical core.In the followings, the method for manufacturing the golf ball will bedescribed in detail.

First, the step of blending (a) the base rubber and at least (d) thecarboxylic acid and/or the salt thereof to prepare the first masterbatchwill be described. As (a) the base rubber blended in the step ofpreparing the first masterbatch, a commercially available base rubbermay be directly used for the blending without any treatment, but thebase rubber is preferably masticated before the blending. “Masticating”means a basic operation in which a mechanical force is applied to a baserubber to disentangle the molecular aggregation or break the molecularchain, thereby adjusting the degree of plasticity of the rubber to sucha level that the rubber is easily processed. The mastication ispreferably conducted by using a known kneading machine such as a rollmill (open roll mill) or a kneader. The temperature of the base rubberduring the mastication is preferably 30° C. or more, and more preferably40° C. or more, and is preferably 100° C. or less, and more preferably90° C. or less. In addition, the time for masticating the base rubber ispreferably 0.1 minute or longer, and more preferably 0.5 minute orlonger, and is preferably 12 minutes or shorter, and more preferably 10minutes or shorter. The mastication of (a) the base rubber and theblending of (a) the base rubber and (d) the carboxylic acid and/or thesalt thereof can be conducted sequentially, simultaneously, orcontinuously.

In the step of preparing the first masterbatch, the material temperatureduring the kneading of (a) the base rubber and at least (d) thecarboxylic acid and/or the salt thereof is preferably 90° C. or less,and more preferably 85° C. or less. This is because if the kneadingtemperature (material temperature) exceeds 90° C., (d) the carboxylicacid and/or the salt thereof may not be dispersed homogenously. Thekneading temperature of preparing the first masterbatch is preferably45° C. or more, and more preferably 50° C. or more. If the kneadingtemperature to prepare the first masterbatch is too low, (d) thecarboxylic acid and/or the salt thereof may not be dispersedhomogenously. Further, the kneading time is preferably 0.5 minute orlonger, more preferably 1.0 minute or longer, and is preferably 20minutes or shorter, more preferably 15 minutes or shorter. If thekneading time falls within the above range, (a) the base rubber and (d)the carboxylic acid and/or the salt thereof are dispersed homogenously.It is preferred that (d) the carboxylic acid and/or the salt thereof isblended only in the step of preparing the first masterbatch.

The kneading of (a) the base rubber and at least (d) the carboxylic acidand/or the salt thereof is preferably conducted using a known kneadingmachine such as a roll mill, a kneader, a banbury mixer, or the like. Inlight of enhancing the efficiency of the kneading, the kneader providinga large shear or banbury mixer is preferably used.

In the present invention, “kneading” means mixing and dispersing severalkinds of blending materials having different properties into the baserubber, on the basis of the formulation of the core rubber composition,while applying a mechanical shear force. In addition, the “masterbatch”is an intermediate composition obtained by blending at least some of theblending materials in consideration of the dispersibility, thereactivity, and the workability of the materials to be blended in therubber composition. The use of the masterbatched intermediatecomposition improves the blending workability. For example, with respectto “the blending material A” that is difficult to blend, “the blendingmaterial A” is blended in the rubber at the concentration which ishigher than that of the blending material A contained in the finalrubber composition to prepare the intermediate composition previously.If the previously prepared intermediate composition and the otherblending materials is blended in such a way that the intermediatecomposition is dilueted, the final rubber composition can be preparedwithout any difficulty in blending the blending material A each time.

In the method for manufacturing the golf ball, (a) the base rubber and(d) the carboxylic acid and/or the salt thereof are preferably blendedin the presence of at least one kind of metal-containing components toprepare the first masterbatch. This is because if (a) the base rubberand (d) the carboxylic acid and/or the salt thereof are blended in thepresence of the metal-containing component, (a) the base rubber and (d)the carboxylic acid and/or the salt thereof can be dispersed morehomogenously.

In the step of preparing the first masterbatch in the presence of atleast one kind of metal-containing components, it is preferable that (a)the base rubber and the metal-containing component are blended, andsubsequently (d) the carboxylic acid and/or the salt thereof is blendedwith the obtained mixture. Alternatively, it is preferable that (a) thebase rubber, (d) the carboxylic acid and/or the salt thereof, and themetal-containing component are preferably blended simultaneously.According to a method where (a) the base rubber and (d) the carboxylicacid and/or the salt thereof are blended and subsequently themetal-containing component is blended with the obtained mixture, theblending materials may not be mixed well.

The metal-containing component is not particularly limited, as long asthe metal containing component is a blending material other than (d)component (carboxylic acid and/or the salt thereof) blended in therubber composition and contains a metal. The metal-containing componentincludes, for example, the metal salt of (b) the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms, (e) the metal compoundblended where necessary, and a metal salt of (f) the organic sulfurcompound. The metal-containing component preferably includes the zinccompound.

In the method for manufacturing the golf ball of the present invention,the first masterbatch is preferably prepared in the presence of themetal salt of (b) the α,β-unsaturated carboxylic acid having 3 to 8carbon atoms and/or (e) the metal compound as the metal-containingcomponent. The first masterbatch is more preferably prepared in thepresence of (b) zinc acrylate and/or (e) zinc oxide as themetal-containing component.

In the step of preparing the first masterbatch, the adding amount of (d)the carboxylic acid and/or the salt thereof is preferably 25 parts bymass or more, more preferably 30 parts by mass or more, even morepreferably 35 parts by mass or more, and is preferably 150 parts by massor less, more preferably 140 parts by mass or less, even more preferably130 parts by mass or less with respect to 100 parts by mass of (a) thebase rubber. If the content of (d) the carboxylic acid and/or the saltthereof contained in the first masterbatch is made high, it is possibleto reduce the adding amount of the first masterbatch when preparing thecore rubber composition.

In the step of preparing the first masterbatch, the blending amount ofthe metal-containing component is preferably 10 parts by mass or more,more preferably 20 parts by mass or more, and is preferably less than200 parts by mass, more preferably 180 parts by mass or less withrespect to 100 parts by mass of (a) the base rubber. If the content ofthe metal-containing component falls within the above range, kneadingworkability becomes better.

If (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atomsand/or the metal salt thereof used as the co-crosslinking agent isblended in the step of preparing the first masterbatch, the blendingamount thereof is preferably 10 parts by mass or more, more preferably15 parts by mass or more, even more preferably 20 parts by mass or more,and is preferably 150 parts by mass or less, more preferably 120 pats bymass or less, even more preferably 100 parts by mass or less withrespect to 100 parts by mass of (a) the base rubber.

If (e) the metal compound is blended in the step of preparing the firstmasterbatch, the blending amount thereof is preferably 5 parts by massor more, more preferably 10 parts by mass or more, even more preferably15 parts by mass or more, and is preferably 100 parts by mass or less,more preferably 90 parts by mass or less, even more preferably 80 parsby mass or less with respect to 100 parts by mass of (a) the baserubber.

Next, the step of blending (a) the base rubber and at least (c) thecrosslinking initiator to prepare the second masterbatch will bedescribed. As (a) the base rubber blended in the step of preparing thesecond masterbatch, a commercially available base rubber may be directlyused for the blending without any treatment, but the base rubber ispreferably masticated before the blending. The mastication is preferablyconducted by using a known kneading machine such as a roll mill or akneader. The temperature of the base rubber during the mastication ispreferably 30° C. or more, more preferably 40° C. or more, and ispreferably 100° C. or less, more preferably 90° C. or less. In addition,the time for masticating the base rubber is preferably 0.1 minute orlonger, more preferably 0.5 minute or longer, and is preferably 12minutes or shorter, more preferably 10 minutes or shorter. Themastication of (a) the base rubber and the blending of (a) the baserubber and (c) the crosslinking initiator can be conducted sequentially,simultaneously, or continuously.

In the step of preparing the second masterbatch, the materialtemperature during the kneading of (a) the base rubber and at least (c)the crosslinking initiator is preferably 95° C. or more, and morepreferably 100° C. or more. If the kneading temperature (materialtemperature) is less than 95° C., the core may not show the requiredperformance. The kneading temperature in preparing the secondmasterbatch is preferably 125° C. or less, and more preferably 120° C.or less. If the kneading temperature in preparing the second masterbatchis too high, the rubber may be scorched. In addition, the kneading timeis preferably 1 minute or longer, more preferably 1.5 minutes or longer,and is preferably 15 minutes or shorter, more preferably 10 minutes orshorter. If the kneading time is within the above range, the blendingmaterials are sufficiently dispersed.

The kneading of (a) the base rubber and at least (c) the crosslinkinginitiator is preferably conducted using a known kneading machine such asa roll mill, a kneader, a banbury mixer, or the like. In light ofenhancing the efficiency of the kneading, the kneader providing a largeshear force or banbury mixer is preferably used.

In the step of preparing the second masterbatch, the adding amount of(c) the crosslinking initiator is preferably 0.3 part by mass or more,more preferably 0.5 part by mass or more, and is preferably 3.0 parts bymass or less, more preferably 2.5 parts by mass or less with respect to100 parts by mass of (a) the base rubber. If the adding amount of the(c) crosslinking initiator falls within the above range, a necessarycrosslinking reaction occurs to provide required performance.

In the step of preparing the second masterbatch, (b) the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms and/or the metal saltthereof; (e) the metal compound and (f) the organic sulfur compoundwhich are added where necessary; or the like may be blended in additionto (a) the base rubber and (c) the crosslinking initiator. It ispreferred that (d) the carboxylic acid and/or the salt thereof isblended only in the step of preparing the first masterbatch.

The embodiments of blending the blending materials constituting the corerubber composition in the steps of preparing the first masterbatch andsecond masterbatch include the following embodiments.

(i) Embodiment which comprises blending (a) the base rubber, (d) thecarboxylic acid and/or the salt thereof, (b) the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereofas the co-crosslinking agent, and (e) the metal compound in the step ofpreparing the first masterbatch; and blending (a) the base rubber, (c)the crosslinking initiator, (b) the α,β-unsaturated carboxylic acidhaving 3 to 8 carbon atoms and/or the metal salt thereof as theco-crosslinking agent, and where necessary (e) the metal compound and/or(f) the organic sulfur compound, in the step of preparing the secondmasterbatch;

(ii) Embodiment which comprises blending (a) the base rubber, (d) thecarboxylic acid and/or the salt thereof, and the metal salt of (b) theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as theco-crosslinking agent in the step of preparing the first masterbatch;and blending (a) the base rubber, (c) the crosslinking initiator, (b)the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/orthe metal salt thereof as the co-crosslinking agent, and where necessary(e) the metal compound and/or (f) the organic sulfur compound, in thestep of preparing the second masterbatch; and

(iii) Embodiment which comprises blending (a) the base rubber, (d) thecarboxylic acid and/or the salt thereof, and (e) the metal compound inthe step of preparing the first masterbatch; and blending (a) the baserubber, (c) the crosslinking initiator, (b) the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereofas the co-crosslinking agent, and where necessary (e) the metal compoundand/or (f) the organic sulfur compound, in the step of preparing thesecond masterbatch.

Next, the step of blending the first masterbatch and the secondmasterbatch to prepare the core rubber composition will be described. Inthe step of preparing the core rubber composition, the materialtemperature during the kneading of the first masterbatch and the secondmasterbatch is preferably 90° C. or less, and more preferably 85° C. orless. If the kneading temperature (material temperature) exceeds 90° C.,the core rubber composition may be attached to a kneading machine,resulting in significant reduction of the working efficiency. Thekneading temperature is preferably 30° C. or more, and more preferably40° C. or more. If the kneading temperature is too low, the insufficientdispersion may occur. In addition, the kneading time is preferably 1minute or longer, more preferably 2 minutes or longer, and is preferably15 minutes or shorter, more preferably 10 minutes or shorter. If thekneading time falls within the above range, the dispersion becomeshomogenous and a variation in physical properties is reduced.

The kneading of the first masterbatch and second masterbatch may beconducted using a known kneading machine, and preferably conducted by aroll mill. Use of the roll mill prevents an excessive raise in thematerial temperature and attachment of a powder to the kneading machine,thereby significantly improving workability.

The blending ratio of the first masterbatch and second masterbatch maybe determined appropriately in accordance with the composition of thefinal core rubber composition; however, the blending ratio (firstmasterbatch/second masterbatch) (mass ratio) of the first masterbatch tothe second masterbatch preferably ranges from 1/99 to 50/50, morepreferably from 5/95 to 30/70.

The step of molding the core rubber composition into the spherical corewill be described. The core rubber composition obtained after kneadingis extruded with an extruder into a bar shape and cut in a predeterminedlength to produce a preform (also referred to as “plug”). Alternatively,the core rubber composition may be formed into a thick sheet shape andstamped out to obtain a plug. The size of each plug may be changed asappropriate in accordance with the size of a mold for compressionmolding. Preferably, the obtained plugs are immersed, for example, in ananti-blocking agent solution such that the plugs are not attached toeach other, dried, and then are matured for about 8 to 48 hours. Next,each plug is placed into the mold for core molding and press-molded. Thematerial temperature during the molding of the spherical core ispreferably 120° C. or more, more preferably 150° C. or more, furtherpreferably 160° C. or more, and is preferably 170° C. or less. If thematerial temperature during the molding exceeds 170° C., the coresurface hardness tends to decrease. In addition, the pressure during themolding is preferably 2.9 MPa to 11.8 MPa. The molding time ispreferably from 10 minutes to 60 minutes.

The method for manufacturing the golf ball of the present inventioncomprises the step of forming at least one cover layer covering thespherical core. An embodiment for molding a cover includes an embodimentwhich comprises injection molding the cover composition containing aresin component directly onto the core, or an embodiment which comprisesmolding the cover composition containing a resin component into ahollow-shell, covering the core with a plurality of the hollow-shellsand subjecting the core with a plurality of the hollow shells to thecompression-molding (preferably an embodiment which comprises moldingthe cover composition into a half hollow-shell, covering the core withthe two half hollow-shells, and subjecting the core with the two halfhollow-shells to the compression-molding).

Examples of the resin component contained in the cover compositioninclude, for example, an ionomer rein; a thermoplastic polyurethaneelastomer having a commercial name of “Elastollan” commerciallyavailable from BASF Japan Ltd; a thermoplastic polyamide elastomerhaving a commercial name of “Pebax” commercially available from ArkemaK. K.; a thermoplastic polyester elastomer having a commercial name of“Hytrel” commercially available from Du Pont-Toray Co., Ltd.; and athermoplastic styrene elastomer having a commercial name of “Rabalon”commercially available from Mitsubishi Chemical Corporation; and thelike.

The ionomer resin includes a product prepared by neutralizing at least apart of carboxyl groups in the binary copolymer composed of an olefinand an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms with ametal ion, a product prepared by neutralizing at least a part ofcarboxyl groups in the ternary copolymer composed of an olefin, anα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, and anα,β-unsaturated carboxylic acid ester with a metal ion, or a mixture ofthose. The olefin preferably includes an olefin having 2 to 8 carbonatoms. Examples of the olefin are ethylene, propylene, butene, pentene,hexene, heptene, and octene. The olefin more preferably includesethylene. Examples of the α,β-unsaturated carboxylic acid having 3 to 8carbon atoms are acrylic acid, methacrylic acid, fumaric acid, maleicacid and crotonic acid. Of these, acrylic acid and methacrylic acid areparticularly preferred. Examples of the α,β-unsaturated carboxylic acidester include methyl ester, ethyl ester, propyl ester, n-butyl ester,isobutyl ester of acrylic acid, methacrylic acid, fumaric acid, maleicacid or the like. In particular, acrylic acid ester and methacrylic acidester are preferable. Of these, the ionomer resin preferably includesthe metal ion-neutralized product of the binary copolymer composed ofethylene-(meth)acrylic acid and the metal ion-neutralized product of theternary copolymer composed of ethylene, (meth)acrylic acid, and(meth)acrylic acid ester.

Specific examples of the ionomer resins include trade name “Himilan(registered trademark) (e.g. the binary copolymerized ionomer such asHimilan 1555 (Na), Himilan 1557 (Zn), Himilan 1605 (Na), Himilan 1706(Zn), Himilan 1707 (Na), Himilan AM3711 (Mg); and the ternarycopolymerized ionomer such as Himilan 1856 (Na), Himilan 1855 (Zn))”commercially available from Du Pont-Mitsui Polychemicals Co., Ltd.

Further, examples include “Surlyn (registered trademark) (e.g. thebinary copolymerized ionomer such as Surlyn 8945 (Na), Surlyn 9945 (Zn),Surlyn 8140 (Na), Surlyn 8150 (Na), Surlyn 9120 (Zn), Surlyn 9150 (Zn),Surlyn 6910 (Mg), Surlyn 6120 (Mg), Surlyn 7930 (Li), Surlyn 7940 (Li),Surlyn AD8546 (Li); and the ternary copolymerized ionomer such as Surlyn8320 (Na), Surlyn 9320 (Zn), Surlyn 6320 (Mg), HPF 1000 (Mg), HPF 2000(Mg))” commercially available from E.I. du Pont de Nemours and Company.

Further, examples include “Iotek (registered trademark) (e.g. the binarycopolymerized ionomer such as Iotek 8000 (Na), Iotek 8030 (Na), Iotek7010 (Zn), Iotek 7030 (Zn); and the ternary copolymerized ionomer suchas Iotek 7510 (Zn), Iotek 7520 (Zn))” commercially available fromExxonMobil Chemical Corporation.

It is noted that Na, Zn, Li, and Mg described in the parentheses afterthe trade names indicate metal types of neutralizing metal ions for theionomer resins. The ionomer resins may be used alone or as a mixture ofat least two of them.

The cover composition constituting the cover of the golf ball of thepresent invention preferably includes, as a resin component, athermoplastic polyurethane elastomer or an ionomer rein. In case ofusing the ionomer rein, it is preferred to use a thermoplastic styreneelastomer together. The content of the polyurethane or ionomer resin ina resin component of the cover composition is preferably 50 mass % ormore, more preferably 60 mass % or more, even more preferably 70 mass %or more.

In the present invention, the cover composition may further contain apigment component such as a white pigment (for example, titanium oxide),a blue pigment, and a red pigment; a weight adjusting agent such as zincoxide, calcium carbonate, and barium sulfate; a dispersant; anantioxidant; an ultraviolet absorber; a light stabilizer; a fluorescentmaterial or a fluorescent brightener; and the like, as long as they donot impair performance of the cover.

The amount of the white pigment (for example, titanium oxide) ispreferably 0.5 part or more, more preferably 1 part or more, and thecontent of the white pigment is preferably 10 parts or less, morepreferably 8 parts or less, with respect to 100 parts by mass of theresin component constituting the cover. If the amount of the whitepigment is 0.5 part by mass or more, it is possible to impart theopacity to the resultant cover. Further, if the amount of the whitepigment is more than 10 parts by mass, the durability of the resultantcover may deteriorate.

When molding the cover in a compression molding method, molding of thehalf shell can be performed by either compression molding method orinjection molding method, and the compression molding method ispreferred. The compression-molding of the cover composition into halfshell can be carried out, for example, under a pressure of 1 MPa or moreand 20 MPa or less at a temperature of −20° C. or more and 70° C. orless relative to the flow beginning temperature of the covercomposition. By performing the molding under the above conditions, ahalf shell having a uniform thickness can be formed. Examples of amethod for molding the cover using half shells include compressionmolding by covering the core with two half shells. The compressionmolding of half shells into the cover can be carried out, for example,under a pressure of 0.5 MPa or more and 25 MPa or less at a temperatureof −20° C. or more and 70° C. or less relative to the flow beginningtemperature of the cover composition. By molding under the aboveconditions, a golf ball cover having a uniform thickness can be formed.

In the case of directly injection molding the cover composition, thecover composition extruded in the pellet form beforehand may be used forinjection molding or the materials such as the base resin components andthe pigment may be dry blended, followed by directly injection moldingthe blended material. It is preferred to use upper and lower moldshaving a spherical cavity and pimples for forming a cover, wherein apart of the pimples also serves as a retractable hold pin. When moldingthe cover by injection molding, the core is placed in the mold, heldwith the protruded hold pin, and the cover composition which has beenheated and melted is charged and then cooled to obtain a cover. Forexample, it is preferred that the cover composition heated and melted atthe temperature ranging from 200° C. to 250° C. is charged into a moldheld under the pressure of 9 MPa to 15 MPa for 0.5 second to 5 seconds,and after cooling for 10 seconds to 60 seconds, the mold is opened.

After the cover is molded, the mold is opened and the golf ball body istaken out from the mold, and where necessary the golf ball body ispreferably subjected to surface treatments such as deburring, cleaning,and sandblast. If desired, a paint film or a mark may be formed.

The golf ball construction of the inventive golf ball is not limited, aslong as the golf ball has a spherical core and at least one cover layercovering the spherical core. The spherical core preferably has a singlelayered structure. Unlike the multi-layered structure, the sphericalcore of the single layered structure does not have an energy loss at theinterface of the multi-layered structure when hitting, and thus has animproved resilience. The cover has a structure of at least one layer,for example a single layered structure, or a multi-layered structure ofat least two layers.

The golf ball of the present invention includes, for example, atwo-piece golf ball comprising a spherical core and a single layeredcover disposed around the spherical core, a multi-piece golf ballcomprising a spherical core, and at least two cover layers disposedaround the spherical core (including the three-piece golf ball), and awound golf ball comprising a spherical core, a rubber thread layer whichis formed around the spherical core, and a cover disposed over therubber thread layer. The present invention can be suitably applied toany one of the above golf balls. If the golf ball of the presentinvention has a multi-layer cover, the cover disposed between theoutermost layer and the spherical core may be referred to as anintermediate layer or inner layer cover (inner cover).

FIG. 1 is a partially cutaway sectional view showing the golf ball 2according to the preferable embodiment of the present invention. Thegolf ball 2 comprises a spherical core 4, and a cover 12 covering thespherical core 4. Plurality of dimples 14 are formed on a surface of thecover. Other portions than dimples 14 on the surface of the golf ball 2are referred to as “land 16”. The golf ball 2 is provided with a paintlayer and a mark layer outside the cover 12, but these layers are notdepicted.

The spherical core preferably has such a hardness distribution that R²of a linear approximate curve determined by a least-squares method is0.95 or more, when plotting JIS-C hardness measured at a center, asurface and at intervals of 2.5 mm from the center of the sphericalcore, versus distances from the center of the spherical core. If R² is0.95 or higher, it means that the hardness distribution of the sphericalcore is linear or almost linear. A golf ball with a spherical corehaving a linear or almost linear hardness distribution exhibits areduced spin rate upon driver shots and middle iron shots, therebyproviding a greater flight distance.

The spherical core was cut into two hemispheres to obtain a cut plane,and the hardness of the spherical core were measured at the centralpoint and at intervals of 2.5 mm from the central point along thearbitrary radius of the spherical core. Although the number of measuringpoints changes depending upon the radius of the spherical core, thehardness distribution of the whole core is obtained by measuring thehardness at intervals of 2.5 mm. Further, the hardness at the surface ofthe spherical core is measured. Next, the JIS-C hardness measured asdescribed above is assigned to the vertical axis and the distance (mm)from the core center is assigned to the horizontal axis, and measurementresults are plotted therein to create a graph. In the present invention,R² of a linear approximation curve obtained from this plot by the leastsquare method is preferably 0.95 or higher. R² of a linear approximationcurve obtained by the least square method is an index representing thelinearity of an obtained plot. In the present invention, if R² is 0.95or higher, it means that the hardness distribution of the spherical coreis linear or almost linear. A golf ball with a spherical core having alinear or almost linear hardness distribution exhibits a reduced spinrate upon driver shots. As a result, a flight distance on driver shotsincreases. R² of the linear approximation curve is preferably 0.96 orhigher. Increasing the linearity provides a greater flight distance ondriver shots.

The spherical core preferably has a hardness difference (Hs−Ho) betweena surface hardness Hs and a center hardness Ho of 18 or more, morepreferably 20 or more, even more preferably 22 or more, and preferablyhas a hardness difference of 80 or less, more preferably 70 or less,even more preferably 60 or less in JIS-C hardness. If the hardnessdifference between the center hardness and the surface hardness islarge, the golf ball having a great flight distance due to the highlaunch angle and low spin rate is obtained. On the other hand, if thehardness difference is too large, the durability of the obtained golfball may be lowered.

The spherical core preferably has the center hardness Ho of 30 or more,more preferably 40 or more, even more preferably 45 or more in JIS-Chardness. If the center hardness Ho is less than 30 in JIS-C hardness,the core becomes too soft and thus the resilience may be lowered.Further, the spherical core preferably has the center hardness Ho of 70or less, more preferably 65 or less, even more preferably 60 or less inJIS-C hardness. If the center hardness Ho exceeds 70 in JIS-C hardness,the core becomes too hard and thus the shot feeling tends to be lowered.

The spherical core preferably has the surface hardness Hs of 72 or more,more preferably 74 or more, even more preferably 76 or more, andpreferably has the surface hardness Hs of 100 or less, more preferably95 or less in JIS-C hardness. If the surface hardness is 76 or more inJIS-C hardness, the spherical core does not become excessively soft, andthus the better resilience is obtained. Further, if the surface hardnessof the spherical core is 100 or less in JIS-C hardness, the sphericalcore does not become excessively hard, and thus the better shot feelingis provided.

The spherical core preferably has the diameter of 34.8 mm or more, morepreferably 36.8 mm or more, and even more preferably 38.8 mm or more,and preferably has the diameter of 42.2 mm or less, more preferably 41.8mm or less, and even more preferably 41.2 mm or less, and mostpreferably 40.8 mm or less. If the spherical core has the diameter of34.8 mm or more, the thickness of the cover does not become too thickand thus the resilience becomes better. On the other hand, if thespherical core has the diameter of 42.2 mm or less, the thickness of thecover does not become too thin, and thus the cover functions better.

When the spherical core has a diameter from 34.8 mm to 42.2 mm, acompression deformation amount (shrinking deformation amount of thespherical core along the compression direction) of the spherical corewhen applying a load from 98 N as an initial load to 1275 N as a finalload is preferably 2.0 mm or more, more preferably 2.8 mm or more, andis preferably 6.0 mm or less, more preferably 5.0 mm or less. If thecompression deformation amount is 2.0 mm or more, the shot feeling ofthe golf ball becomes better. If the compression deformation amount is6.0 mm or less, the resilience of the golf ball becomes better.

In the present invention, the thickness of the cover of the golf ball ispreferably 4.0 mm or less, more preferably 3.0 mm or less, even morepreferably 2.0 mm or less. If the thickness of the cover is 4.0 mm orless, the resilience and shot feeling of the obtained golf ball becomebetter. The thickness of the cover is preferably 0.3 mm or more, morepreferably 0.5 mm or more, and even more preferably 0.8 mm or more, andmost preferably 1.0 mm or more. If the thickness of the cover is lessthan 0.3 mm, the durability and the wear resistance of the cover maydeteriorate. In case of a plurality of cover layers, it is preferredthat the total thickness of the cover layers falls within the aboverange.

The slab hardness of the cover composition is preferably determined inaccordance with the desired performance of the golf balls. For example,in case of a so-called distance golf ball which focuses on a flightdistance, the cover composition constituting the outermost cover layer(hereinafter, sometimes may be merely referred to as “outermost coverlayer composition”) preferably has a slab hardness of 50 or more, morepreferably 55 or more, and preferably has a slab hardness of 80 or less,more preferably 70 or less in Shore D hardness. If the outermost coverlayer composition has a slab hardness of 50 or more, the obtained golfball has a high launch angle and low spin rate on driver shots and ironshots, and thus the flight distance becomes large. If the outermostcover layer composition has a slab hardness of 80 or less, the golf ballexcellent in durability is obtained. Further, in case of a so-calledspin golf ball which focuses on controllability, the outermost coverlayer composition preferably has a slab hardness of less than 50, andpreferably has a slab hardness of 20 or more, more preferably 25 or morein Shore D hardness. If the outermost cover layer composition has a slabhardness of less than 50, the flight distance on driver shots can beimproved by the core of the present invention, as well as the obtainedgolf ball readily stops on the green due to the high spin rate onapproach shots. If the outermost cover layer composition has a slabhardness of 20 or more, the abrasion resistance improves.

In case of a plurality of cover layers, the cover compositionconstituting the intermediate layer or inner cover layer (hereinafter,sometimes may be merely referred to as “inner cover layer composition”)preferably has a slab hardness of 40 or more, more preferably 45 ormore, more preferably 48 or more, and preferably has a slab hardness of80 or less, more preferably 75 or less, even more preferably 70 or lessin Shore D hardness. If the slab hardness of the inner layer covercomposition is 40 or more in Shore D hardness, the rigidity of theintermediate layer or inner cover layer enhances and thus the golf ballwith an excellent resilience is obtained. If the slab hardness of theinner cover layer composition is 80 or less in Shore D hardness, thedurability of the obtained golf ball improves.

The concave portions called “dimple” are usually formed on the surfaceof the cover. The total number of the dimples is preferably 200 or moreand 500 or less. If the total number is less than 200, the dimple effectis hardly obtained. On the other hand, if the total number exceeds 500,the dimple effect is hardly obtained because the size of the respectivedimples is small. The shape (shape in a plan view) of dimples includes,for example, without limitation, a circle, polygonal shapes such asroughly triangular shape, roughly quadrangular shape, roughly pentagonalshape, roughly hexagonal shape, and another irregular shape. The shapeof the dimples is employed solely or at least two of them may be used incombination.

It is preferred that a paint film is formed on a surface of the golfball body. The paint film preferably has a thickness of, but not limitedto, 5 μm or larger, and more preferably 7 μm or larger, and preferablyhas a thickness of 50 μm or smaller, and more preferably 40 μm orsmaller, even more preferably 30 μm or smaller. If the thickness issmaller than 5 μm, the paint film is easy to wear off due to continueduse of the golf ball, and if the thickness is larger than 50 μm, theeffect of the dimples is reduced, resulting in lowering flyingperformance of the golf ball.

When the golf ball of the present invention has a diameter in a rangefrom 40 mm to 45 mm, a compression deformation amount of the golf ball(shrinking amount of the golf ball in the compression direction thereof)when applying a load from an initial load of 98 N to a final load of1275 N to the golf ball is preferably 2.0 mm or more, more preferably2.4 mm or more, even more preferably 2.5 mm or more, most preferably 2.8mm or more, and is preferably 5.0 mm or less, more preferably 4.5 mm orless. If the compression deformation amount is 2.0 mm or more, the golfball does not become excessively hard, and thus exhibits the good shotfeeling. On the other hand, if the compression deformation amount is 5.0mm or less, the resilience is enhanced.

EXAMPLES

Hereinafter, the present invention will be described in detail by way ofexample. The present invention is not limited to examples describedbelow. Various changes and modifications can be made without departingfrom the spirit and scope of the present invention.

[Evaluation Methods]

(1) Compression Deformation Amount (mm)

A compression deformation amount of the core or golf ball (a shrinkingamount of the core or golf ball in the compression direction thereof),when applying a load from 98 N as an initial load to 1275 N as a finalload to the core or golf ball, was measured.

(2) Coefficient of Restitution

A 198.4 g of metal cylindrical object was allowed to collide with eachcore or golf ball at a speed of 40 m/sec, and the speeds of thecylindrical object and the core or golf ball before and after thecollision were measured. Based on these speeds and the mass of eachobject, coefficient of restitution for each core or golf ball wascalculated. The measurement was conducted by using twelve samples foreach core or golf ball, and the average value was regarded as thecoefficient of restitution for the core or golf ball. The coefficient ofrestitution of golf balls (core) No.1, 3 and 4 is shown as thedifference from that of golf ball (core) No.6. The coefficient ofrestitution of golf ball (core) No.2 is shown as the difference fromthat of golf ball No.5. The coefficient of restitution of golf balls(core) No.7 and 8 is shown as the difference from that of golf ball(core) No.9.

(3) Slab Hardness (Shore D Hardness)

Sheets with a thickness of about 2 mm were produced by injection moldingthe cover composition, and stored at 23° C. for two weeks. Three or moreof these sheets were stacked on one another so as not to be affected bythe measuring substrate on which the sheets were placed, and thehardness of the stack was measured with a type P1 auto loading durometermanufactured by Kobunshi Keiki Co., Ltd., provided with a Shore D typespring hardness tester prescribed in ASTM-D2240.

(4) Hardness Distribution of Spherical Core (JIS-C Hardness)

A type P1 auto loading durometer manufactured by Kobunshi Keiki Co.,Ltd., provided with a JIS-C type spring hardness tester was used tomeasure the hardness of the spherical core. The hardness measured at thesurface of the spherical core was adopted as the surface hardness of thespherical core. The spherical core was cut into two hemispheres toobtain a cut plane, and the hardness were measured at the central pointand at predetermined distances from the central point. The core hardnesswere measured at 4 points at predetermined distances from the centralpoint of the cut plane of the core. The core hardness was calculated byaveraging the hardness measured at 4 points.

(5) Flight Distance (m) and Spin Rate (rpm) on a Driver Shot

A metal-headed W#1 driver (XXIO S, loft: 11°, manufactured by SRI SportsLimited) was installed on a swing robot M/C manufactured by GolfLaboratories, Inc. A golf ball was hit at a head speed of 40 m/sec, andthe flight distance (the distance from the launch point to the stoppoint) and the spin rate right after hitting the golf ball weremeasured. This measurement was conducted twelve times for each golfball, and the average value was adopted as the measurement value for thegolf ball. A sequence of photographs of the golf ball right afterhitting were taken for measuring the spin rate (rpm). The flightdistance and spin rate of golf balls (core) No.1, 3 and 4 are shown asthe difference from those of golf ball (core) No.6. The flight distanceand spin rate of golf ball (core) No.2 are shown as the difference fromthose of golf ball (core) No.5. The flight distance and spin rate ofgolf balls (core) No.7 and 8 are shown as the difference from those ofgolf ball (core) No.9.

(6) Kneading Workability

Kneading workability was determined on the basis of the followingcriterion (visual observation).

G (Good): Attachment to the kneading machine is equivalent to that in anordinary formulation (Manufacturing Method No. 18).

P (Poor): Attachment to the kneading machine is obviously larger inamount than that in the ordinary formulation (Manufacturing Method No.18).

[Production of Golf Ball]

(1) Production of Spherical Core

Regarding Manufacturing Method Nos. 1 to 15 for Spherical Core

The blending materials shown in Table 3 were kneaded with a kneader toprepare a first masterbatch and a second masterbatch. In Table 3, theblending material charging procedure (a first step, a second step, and athird step) in the step of preparing the first masterbatch shows theorder of blending the blending materials (1) to (8). For example, inManufacturing Method No. 1, it means that (1) polybutadiene wasmasticated (first step) and then (2) zinc caprylate and (6) zinc oxidewere added at the same time and kneaded. The kneading for the firstmasterbatch was conducted by using a kneader (capacity: 55 L). Withrespect to the conditions for the mastication of the base rubber (firststep), the material temperature was 50° C. and the kneading time was 3minutes. With respect to the conditions for preparing the firstmasterbatch (second step and third step), the material temperature was85° C. or less and the total kneading time was 7 minutes. In the step ofpreparing the second masterbatch, the base rubber was masticated, andthen all the blending materials were kneaded at the same time. Thekneading for the second masterbatch was conducted by using a kneader(capacity: 55 L). With respect to the conditions for the mastication ofthe base rubber, the material temperature was 50° C. and the kneadingtime was 3 minutes. With respect to the conditions for preparing thesecond masterbatch (the kneading of the blending materials), thematerial temperature was 105° C. and the kneading time was 4 minutes.

The first masterbatch was added in the predetermined amount shown inTable 3 to the obtained second masterbatch and kneaded to prepare thecore rubber composition. The kneading of the first masterbatch and thesecond masterbatch was conducted by using a roll mill (roll diameter: 22inches) at a temperature of 75° C. for 6 minutes. The obtained corerubber composition was extruded using an extruder to prepare a plug. Theobtained plug was placed and heat-pressed in upper and lower molds, eachhaving a hemispherical cavity, at 170° C. for 20 minutes to prepare thespherical cores.

TABLE 3 Spherical core manufacturing method No. 1 2 3 4 5 6 7 8 Step ofComposition 1. Polybutadiene rubber 100 100 100 100 100 100 100 100preparing (parts by mass) 2. Zinc caprylate 25 25 25 25 50 50 50 100first 3. Decanoic acid — — — — — — — — masterbatch 4. Zinc Laurate — — —— — — — — 5. Zinc stearate — — — — — — — — 6. Zinc oxide 25 — — — 50 — —— 7. Barium sulfate — — — — — — — — 8. Zinc acrylate — 25 25 25 — 50 50100 Blending material First step 1 1 1 1 1 1 1 1 charging Second step2.6 2.8 8 8 2.6 2.8 8 8 procedure Third step — — 2 2 — — 2 2 Kneadingtemperature (° C.) 85 85 85 85 85 85 85 85 Kneading workability G G G GG G G G Step of Composition Polybutadiene rubber 80 80 80 80 90 90 90 95preparing (parts by mass) Zinc acrylate 35 25 25 30 35 30 30 30 secondZinc oxide 0 5 5 5 0 5 5 5 masterbatch Barium sulfate *1) *1) *1) *1)*1) *1) *1) *1) Dicumyl peroxide 0.9 0.8 0.8 0.9 0.9 0.9 0.9 0.92-thionaphthol 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 2,6-dichlorothiophenol —— — — — — — — Kneading temperature (° C.) 105 105 105 105 105 105 105105 Core rubber Parts by mass of first masterbatch 30 30 30 30 20 20 2015 composition Kneading temperature (° C.) 75 75 75 75 75 75 75 75Kneading workability G G G G G G G G Spherical core manufacturing methodNo. 9 10 11 12 13 14 15 Step of Composition 1. Polybutadiene rubber 100100 100 100 100 100 100 preparing (parts by mass) 2. Zinc caprylate 150200 — — 25 50 — first 3. Decanoic acid — — — — — — 50 masterbatch 4.Zinc Laurate — — 50 — — — — 5. Zinc stearate — — — 50 50 — — 6. Zincoxide — — — — — — — 7. Barium sulfate — — — — 10 — — 8. Zinc acrylate150 200 50 50 — 50 50 Blending material First step 1 1 1 1 1 1 1charging Second step 8 8 8 8 7 8 8 procedure Third step 2 2 4 5 2.5 2 3Kneading temperature (° C.) 85 85 85 85 85 85 85 Kneading workability GP G G P G G Step of Composition Polybutadiene rubber 96.7 — 80 80 — 9090 preparing (parts by mass) Zinc acrylate 30 — 20 20 — 22 24 secondZinc oxide 5 — 5 5 — 5 5 masterbatch Barium sulfate *1) — *1) *1) — *1)*1) Dicumyl peroxide 0.9 — 0.8 0.8 — 0.9 0.9 2-thionaphthol 0.2 — 0.20.2 — — — 2,6-dichlorothiophenol — — — — — 0.11 0.36 Kneadingtemperature (° C.) 105 — 105 105 — 105 105 Core rubber Parts by mass offirst masterbatch 13.3 — 40 40 — 20 20 composition Kneading temperature(° C.) 75 — 75 75 — 75 75 Kneading workability G — G G — G G *1) As toan amount of barium sulfate, adjustment was made such that the golf ballhad a mass of 45.4 g.

Regarding Manufacturing Method Nos. 16 to 19 for Spherical Core

Blending materials shown in Table 4 were kneaded to prepare a corerubber composition. The components other than a crosslinking initiator(dicumyl peroxide) were added into the masticated base rubber(polybutadiene), and the kneading was conducted sufficiently. Then, thecrosslinking initiator was added to the mixture, and the kneading wasconducted again. The kneading was conducted using a kneader (capacity:55 L) at the temperature of 110° C. and the total kneading time was for3 minutes.

TABLE 4 Spherical core manufacturing method No. 16 17 18 19 Step ofmolding Composition Polybutadiene 100 100 100 100 spherical core (partsby mass) Zinc caprylate — — — 5 rubber composition Decaonic acid — — — —Zinc laurate — — — — Zinc stearate — — — — Zinc oxide 5 5 5 5 Bariumsulfate *1) *1) *1) *1) Zinc acrylate 30 27 23 30 2-thiophenol 0.2 0.2 —0.2 Dicumyl peroxide 0.9 0.8 0.8 0.8 Kneading temperature (° C.) 110 110110 110 Kneading workability G G Control P *1) As to an amount of bariumsulfate, adjustment was made such that the golf ball had a mass of 45.4g.

Raw materials used in Tables 3 and 4 are follows.

-   Polybutadiene rubber: a high-cis polybutadiene “BR730” (cis-1,4 bond    content=96 mass %, 1,2-vinyl bond content=1.3 mass %, Moony    viscosity (ML₁₊₄ (100° C.)=55, molecular weight distribution    (Mw/Mn)=3) available from JSR Corporation Zinc acrylate: “ZNDA-90S”    manufactured by Nihon Jyoryu Kogyo Co., Ltd.-   Zinc oxide: “Ginrei R” manufactured by Toho Zinc Co., Ltd.-   Barium sulfate: “Barium sulfate BD” manufactured by Sakai Chemical    Industry Co., Ltd., adjustment was made such that the finally    obtained golf ball had a mass of 45.4 g.-   2-thionaphthol: manufactured by Tokyo Chemical Industry Co., Ltd.-   Dicumyl peroxide: “Percumyl (registered trademark) D” manufactured    by NOF Corporation.-   Zinc stearate: manufactured by Wako Pure Chemical Industries    (purity: 99%)-   Zinc laurate: manufactured by Mitsuwa Chemicals Co., Ltd.-   Decanoic acid: manufactured by Tokyo Chemical Industry Co., Ltd.-   Zinc caprylate: manufactured by Mitsuwa Chemicals Co., Ltd.    (2) Preparation of Cover composition

Cover materials shown in Table 5 were mixed with a twin-screw kneadingextruder to prepare the cover compositions in the pellet form. Theextruding conditions of the cover composition were a screw diameter of45 mm, a screw rotational speed of 200 rpm, and screw L/D=35, and themixtures were heated to 160° C. to 230° C. at the die position of theextruder.

TABLE 5 Cover composition No. A B C D Elastollan XNY85A — — — 100 Surlyn 8945 50 34 25 — Himilan AM7329 50 40 50 — Nucrel 1050H — — 25 —Rabalon T3221C — 24 — — Tungsten — 22 — — Ultramarine blue — —    0.04 —Titanium oxide  4 —  3  4 Slab hardness (shore D) 65 50 60 32Formulation: parts by mass(3) Production of Golf Ball Body

The cover composition obtained above was injection-molded onto thespherical cores to form the golf ball cover covering the spherical core.Golf balls No.1 to No.6 are three-piece golf balls comprising aspherical core and a multi-layer cover consisting of an inner coverlayer and outer cover layer disposed around the spherical core. Golfballs No.7 to No.9 are two-piece golf balls comprising a spherical coreand a single-layer cover disposed around the spherical core. Upper andlower molds for the cover have a spherical cavity with pimples, a partof which serves as a hold pin which is extendable and retractable.

When molding the cover, the core was placed and held with the protrudedhold pins, the cover composition heated at the temperature in a rangefrom 210 ° C. to 260° C. was charged into the mold clamped under apressure of 80 tons within 0.3 seconds, and cooled for 30 seconds. Then,the mold was opened, and the golf ball bodies were taken out from themold. The surface of the obtained golf ball bodies were treated withsandblast, marked, and painted with a clear paint. The paint was driedin an oven at 40° C. to form a paint film, and golf balls having adiameter of 42.8 mm and a mass of 45.4 g were obtained. The results ofevaluations of the golf balls were shown in Tables 6 and 7.

TABLE 6 Golf ball No. 1 2 3 4 5 6 Spherical core Polybutadiene rubber100 100 100 100 100 100 rubber composition Zinc caprylate 5 5 — — — —(parts by mass) Zinc laurate — — 10 — — — Zinc stearate — — — 10 — —Decaonic acid — — — — — — Zinc oxide 5 5 5 5 5 5 Zinc acrylate 30 35 3030 30 27 Barium sulfate *1) *1) *1) *1) *1) *1) 2-thiophenol 0.2 0.2 0.20.2 0.2 0.2 2,6-dichlorothiophenol — — — — — — Dicumyl peroxide 0.8 0.90.8 0.8 0.9 0.8 Spherical core manufacturing method No. 2.3 1.4-9 11 1216 17 Spherical core Core center hardness 52.4 51.5 55.0 57.0 61.0 59.0hardness distribution  2.5 mm 57.0 59.0 57.5 57.5 65.3 64.6 (JIS-C)  5.0mm 63.6 63.4 63.6 63.6 70.1 69.8  7.5 mm 65.4 65.0 65.4 65.4 71.0 70.110.0 mm 68.6 70.1 68.6 68.6 72.3 71.2 12.5 mm 70.6 76.8 70.6 70.6 69.169.1 15.0 mm 80.4 79.1 80.4 80.4 75.7 75.7 17.0 mm 84.0 80.1 84.0 84.078.4 78.4 Surface hardness 89.7 82.6 89.0 88.5 85.0 85.9 Surfacehardness − center hardness 37.3 31.1 34.0 31.5 24.0 26.9 R² ofapproximated curve 0.97 0.97 0.97 0.96 0.86 0.87 Slope of approximatedcurve 1.81 1.54 1.71 1.64 0.97 1.07 Spherical core diameter (mm) 39.239.8 39.2 39.2 39.8 39.2 Spherical core coefficient of restitution 0.000.00 0.00 0.000 0.000 0.000 Spherical core compression deformationamount (mm) 4.00 3.30 3.98 3.98 3.28 3.98 Inner cover layer compositionB A B B A B Inner cover layer hardness (Shore D) 50 65 50 50 65 50 Innercover layer thickness (mm) 1 1 1 1 1 1 Outer cover layer composition C DC C D C Outer cover layer hardness (Shore D) 60 32 60 60 32 60 Outercover layer thickness (mm) 0.8 0.5 0.8 0.8 0.5 0.8 EvaluationCompression deformation amount (mm) 3.10 2.80 3.05 3.04 2.79 3.08 Driverspin rate (rpm) −300 −100 −200 −100 0 0 Driver flight distance (m) 3.02.0 2.0 2.0 0.0 0.0

TABLE 7 Golf ball No. 7 8 9 Rubber composition Polybutadiene rubber 100100 100 (parts by mass) Zinc caprylate 5 — — Zinc laurate — — — Zincstearate — — — Decaonic acid — 5 — Zinc oxide 5 5 5 Zinc acrylate 27 2923 Barium sulfate *1) *1) *1) 2-thiophenol — — — 2,6-dichlorothiophenol0.11 0.36 — Dicumyl peroxide 0.9 0.9 0.8 Spherical core manufacturingmethod No. 14 15 18 Core hardness Core center hardness 47.5 48.8 55.0distribution  2.5 mm 55.6 55.9 61.4 (JIS-C)  5.0 mm 63.1 62.2 65.4  7.5mm 67.0 65.3 67.1 10.0 mm 68.8 66.4 67.4 12.5 mm 69.9 67.6 67.2 15.0 mm77.0 73.9 71.3 17.5 mm 79.7 76.5 73.4 Surface hardness 85.3 80.9 80.2Surface hardness − center hardness 37.8 32.1 25.2 R² of approximatedcurve 0.96 0.96 0.90 Slope of approximated curve 1.70 1.44 0.99Spherical core diameter (mm) 39.8 39.8 39.8 Spherical core coefficientof restitution 0.010 0.009 0.000 Spherical core compression deformationamount (mm) 3.88 4.30 4.20 Inner cover layer composition — — — Innercover layer hardness (Shore D) — — — Inner cover layer thickness (mm) —— Outer cover layer composition A A A Outer cover layer hardness (ShoreD) 65 65 65 Outer cover layer thickness (mm) 1.5 1.5 1.5 EvaluationCompression deformation amount (mm) 3.18 3.60 3.50 Driver spin rate(rpm) −150 −80 0 Driver flying distance (m) 4.0 2.8 0.0

As shown in Tables 6 and 7, the method for manufacturing a golf ballthat comprises a spherical core formed from a core rubber compositioncontaining (a) a base rubber, (b) an α,β-unsaturated carboxylic acidhaving 3 to 8 carbon atoms and/or a metal salt thereof as aco-crosslinking agent, (c) a crosslinking initiator, (d) a carboxylicacid and/or a salt thereof, and (e) a metal compound where necessary,and at least one cover layer covering the spherical core, comprising thesteps of: blending (a) the base rubber and at least (d) the carboxylicacid and/or the salt thereof to prepare a first masterbatch; blending(a) the base rubber and at least (c) the crosslinking initiator toprepare a second masterbatch; blending the first masterbatch and thesecond masterbatch to prepare the core rubber composition; molding thecore rubber composition into the spherical core; and forming at leastone cover layer covering the spherical core, is excellent in kneadingworkability. The golf ball obtained by the method has a low spin rate ondriver shots and travels a great flight distance.

The present invention is useful as a method for manufacturing a golfball traveling a great flight distance on driver shots. This applicationis based on Japanese Patent applications No. 2012-113567 filed on May17, 2012, the contents of which are hereby incorporated by reference.

The invention claimed is:
 1. A method for manufacturing a golf ball thatcomprises a spherical core formed from a core rubber compositioncontaining: (a) a base rubber, (b) an α, β-unsaturated carboxylic acidhaving 3 to 8 carbon atoms and/or a metal salt thereof as aco-crosslinking agent, (c) a crosslinking initiator, and (d) acarboxylic acid and/or a salt thereof not including (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or a metalsalt thereof, and at least one cover layer covering the spherical core,comprising the steps of: blending (a) the base rubber and at least (d)the carboxylic acid and/or the salt thereof to prepare a firstmasterbatch; blending (a) the base rubber, and at least (b) the α,β-unsaturated carboxylic acid having 3to 8 carbon atoms and/or a metalsalt thereof and (c) the crosslinking initiator to prepare a secondmasterbatch while adjusting a material temperature thereof to 95° C. ormore; blending the first masterbatch and the second masterbatch toprepare the core rubber composition while adjusting a materialtemperature thereof to 90° C. or less; molding the core rubbercomposition into the spherical core; and forming at least one coverlayer covering the spherical core, wherein the core rubber compositionmay further contain (e) a metal compound to neutralize the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, a weightadjusting agent to adjust the weight of the spherical core, a hardnessadjusting agent to adjust the hardness of the spherical core, or aninorganic pigment.
 2. The method for manufacturing the golf ballaccording to claim 1, wherein (a) the base rubber and at least (d) thecarboxylic acid and/or the salt thereof are kneaded to prepare the firstmasterbatch while adjusting a material temperature thereof to 90° C. orless.
 3. The method for manufacturing the golf ball according to claim2, wherein (d) the carboxylic acid and/or the salt thereof is blended inan amount of 25 parts by mass to 150parts by mass with respect to 100parts by mass of (a) the base rubber in the step of preparing the firstmasterbatch.
 4. The method for manufacturing the golf ball according toclaim 1, wherein the first masterbatch is prepared in the presence of atleast one kind of metal-containing components.
 5. The method formanufacturing the golf ball according to claim 4, wherein the firstmasterbatch is prepared in the presence of the metal salt of (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms which is theco-crosslinking agent and/or (e) the metal compound as themetal-containing component.
 6. The method for manufacturing the golfball according to claim 4, wherein the first masterbatch is prepared inthe presence of a zinc compound as the metal-containing component. 7.The method for manufacturing the golf ball according to claim 4, whereinthe first masterbatch is prepared in the presence of (b) zinc acrylateand/or (e) zinc oxide as the metal-containing component.
 8. The methodfor manufacturing the golf ball according to claim 4, wherein themetal-containing component is contained in an amount of 10 parts by massor more and less than 200 parts by mass with respect to 100 parts bymass of (a) the base rubber.
 9. The method for manufacturing the golfball according to claim 1, wherein (d) the carboxylic acid and/or thesalt thereof is a carboxylic acid having 4 to 30 carbon atoms and/or asalt thereof.
 10. The method for manufacturing the golf ball accordingto claim 1, wherein (d) the carboxylic acid and/or the salt thereof is afatty acid and/or a salt thereof.
 11. The method for manufacturing thegolf ball according to claim 10, wherein the fatty acid is a saturatedfatty acid.
 12. The method for manufacturing the golf ball according toclaim 1, wherein the core rubber composition contains (d) the carboxylicacid and/or the salt thereof in an amount from 0.1 part by mass to 40.0parts by mass with respect to 100 parts by mass of (a) the base rubber.13. The method for manufacturing the golf ball according to claim 1,wherein the spherical core has a hardness distribution that R² of alinear approximate curve determined by a least-squares method is 0.95 ormore, when plotting if JIS-C hardness values are measured at the corecenter, the core surface and at intervals of 2.5 mm from the core centerand plotted versus distances from the core center, then R² of a linearapproximate curve determined by the least-squares method is 0.95 ormore.
 14. The method for manufacturing the golf ball according to claim1, wherein the spherical core has a hardness difference ranging from 18to 80 in JIS-C hardness between a surface hardness Hs and a centerhardness Ho thereof.
 15. The method for manufacturing the golf ballaccording to claim 1, wherein (d) the carboxylic acid and/or the saltthereof is blended only in the step of preparing the first masterbatch.16. The method for manufacturing the golf ball according to claim 1,wherein the first masterbatch and the second masterbatch are blendedwhile adjusting a material temperature thereof to 85° C. or less.